1. Field of Application
The operation relates to a wobble information detection method and apparatus for operating on a wobble signal that is read from an optical recording medium, which in general is an optical recording disk.
In particular, the apparatus relates to a wobble information detection method and apparatus whereby reliable detection can be achieved for information that has been recorded by phase modulation of a wobble that is formed along a track of the optical recording medium.
2. Background of the Invention
There are various standards and recording formats for playback types of optical recording medium such as a CD (compact disk), DVD (digital versatile disk), etc., and for write-once types of optical recording medium such as a CD-R (rewritable CD), DVD-R, etc., and for rewritable types of optical recording medium such as a DVD-RAM (random-access memory DVD), DVD-RW (rewritable DVD), etc.
In the case of a write-once type of optical recording disk such as a CD-R for example, as seen in the partial view of the lower side of such an optical recording disk 50 shown in FIG. 7, a track is formed along a groove 51 which wobbles laterally with a fixed periodic of variation, with that wobble-shaped groove being referred to in the following simply as the “wobble”. A playback signal from an optical pick-up (not shown in the drawing) which reads data from the track also derives a signal (referred to in the following as the wobble signal) from the wobble, with that wobble signal having a specific frequency (standardized as 22.05 kHz). In the following, only playback of the wobble signal will be considered.
The wobble is recorded by being phase modulated with information such as track address information (ATIP), indicating the absolute positions of tracks. In an optical disk recording/playback apparatus (referred to in the following simply as an optical disk apparatus), an optical pick-up forms a focused light spot (indicated by numeral 52 in FIG. 7) onto a track and derives a wobble signal from the resultant reflected light. The wobble signal is then demodulated, and the aforementioned recorded information from the demodulated wobble signal is used by the optical disk apparatus in generating a disk rotation control signal during a recording or playback operation, and to generate a reference clock signal.
In the case of a DVD-R or DVD-RAM, on the other hand, the wobble does not consist of an actual shape variation of the tracks, with the wobble information instead being recorded as pits on the land side of a track. With such a method, no modulation is applied to the wobble, and the wobble signal is used to set a clock frequency (140 kHz) for disk rotation control.
Considering the wobble that is recorded by the phase modulation method, the phase is inverted to indicate respective bit states (e.g., with the 0° phase indicating the “1” bit state, and the 180° phase indicating the “0” bit state). The optical disk apparatus applies phase discrimination to the wobble signal to thereby demodulate the aforementioned recorded information.
One method of demodulation of the wobble signal is described for example in Japanese patent Laid-open No. 2002-208231 (page 2, FIG. 7). With that method, a threshold value is established with respect to the wobble signal, for use in detecting the phase inversions. By judging the number of times that the wobble signal is inverted within a fixed unit interval, the “0” and “1” bit states can be discriminated, as illustrated in the example of FIGS. 8A, 8B in which phase modulation of a wobble signal expresses a “0” and a “1” bit, respectively, with the modulation performed in units of 8 periods of the wobble signal. By using two threshold values designated as Vt1, Vt2 respectively for threshold level comparison, the number of phase inversions which occur within a unit interval are detected to thereby obtain a pulse waveform as shown. Based on the time relationship between two of these pulses that occur within a unit interval, a decision is made as to whether a “0” or a “1” bit is expressed in that unit data section.
An alternative method of using a wobble signal has been proposed in Japanese patent Laid-open No. 2001-209937, whereby each of successive unit intervals of the wobble signal is modulated to have either a 0° or 180° phase, in accordance with whether the interval expresses the “1” or the “0” bit state. In the following, such phase modulation intervals of the wobble signal are referred to as unit data sections. During playback of the wobble signal, a sampling clock signal is generated which is locked in phase and frequency with the playback wobble signal, and which is used to generate sampling pulses for use in performing synchronous detection of the wobble signal, i.e., by sampling successive periods of that signal. The resultant sample values that are obtained for a unit data section are successively integrated, to obtain a final value (phase integration value), whose amplitude is indicative of the phase of the wobble signal within that unit data section. In that way, discrimination of the “1” and “0” bit states expressed in the playback wobble signal can be performed based on the levels of the phase integration values obtained for respective unit data sections.
This method is illustrated in the timing diagram example of FIG. 9, in which in addition to the aforementioned sampling pulses, information detection timing pulses are derived from synchronizing information (i.e., a synchronizing pattern) recorded on the optical disk. The information detection timing pulses define respective intervals in which integration is performed of the sample values that are obtained for one unit data section. At the end of each information detection timing pulse, a decision is made as to whether a “0” or a “1” bit is expressed, based on the phase integration output level which has been attained at that time.
In addition to two unit data sections which express the “1” and “0” bit states respectively, the diagram of FIG. 9 shows a reference wobble section of the wobble signal, whose function will be described hereinafter.
However as increasingly high values of recording density are achieved for optical recording media, so that the track pitch is reduced, the signal/noise ratio of the playback signal becomes lowered. Specifically, if phase modulation of the wobble signal is utilized as described above, then as the track pitch is reduced, there is an increasing degree of crosstalk in the playback wobble signal, due to the wobble of adjacent tracks. This results in deterioration of the wobble signal and phase inversions of the wobble signal may also occur, so that the accuracy of synchronous detection of the playback wobble signal is reduced, and the accuracy of demodulation may thereby be reduced.
These problem become more severe if there is any deterioration of the optical recording medium.
It should be noted that such problems are not limited to methods in which modulation is performed by alteration of the phase within respective unit intervals. The waveform diagram of FIG. 9 illustrates a method whereby a fundamental wobble signal frequency has a fixed proportion of a component at twice the fundamental frequency added thereto, or subtracted therefrom, to express the “1” and “0” bit states. However with such a method, crosstalk of the superimposed frequency component having twice the fundamental frequency may occur from adjacent tracks, and result in errors in detection of the “1” and “0” bit states from the playback wobble signal.
In an attempt to reduce this problem, in the case of the phase modulation method illustrated in FIG. 9, sections referred to in the following as reference wobble sections are provided in the recorded wobble, with each reference wobble section immediately preceding a sequence of unit data sections which constitute actual recorded information, such a sequence being referred to in the following as a data wobble sequence. A reference phase integration value is obtained from each reference wobble section in the playback wobble signal, corresponding to a specific predetermined bit state. Thus by comparing each (final) phase integration value that is obtained in the unit data sections of a data wobble sequence with the reference phase integration value of the preceding reference wobble section, it becomes possible to more accurately discriminate between the “1” and “0” bit states.
However it is found in practice that such a countermeasure, in itself, is not sufficient to overcome the above-mentioned problem of insufficient accuracy of demodulating the playback wobble signal, i.e., problems in accurately discriminating between the “1” and “0” bit states expressed in respective unit intervals of the wobble signal, arising from increasing recording density on the optical recording medium. As illustrated in the example of FIG. 9, there is an offset in the level values that are obtained for the reference phase value and for the reference phase values that are obtained in the unit data sections of the data wobble sequence and this, together with the problem of crosstalk that produces distortion of the playback wobble signal and phase inversions in that signal, makes it difficult to achieve accurate demodulation of the information that has been recorded by phase modulation of the wobble.